1. Field of the Invention
This invention relates to the manufacture of magnetic cores for power transformers and particularly to the manufacture of single and three-phase magnetic cores for small to medium power transformers used mainly in electrical energy distribution networks.
2. Description of the Prior Art
The magnetic core is one of the two necessary elements of any transformer, the other being the windings. There are two main requirements that the magnetic core must satisfy:
(a) A closed path for the magnetic flux generated in the core by the AC current in the windings.
(b) A minimum loss of power due to the core re-magnetization process.
The most obvious solution to satisfy the first requirement is to provide a toroid-shaped core made from a continuous ribbon of magnetic strip material. However, this is not acceptable in most practical cases due to the complexity of placing the windings onto a closed core. In practice, the coil/core assembly problem is solved by making a core with one or more special joints, which are used to open up the core loops, place the windings (or wind them directly onto the straight parts of the core) and then close the loops. Joint designs with minimum resistance to magnetic flux flow throughout the core have been developed and implemented in manufacturing practice.
Minimum power loss in a core is achieved by:
(a) Making the core from a soft magnetic material (typically xe2x88x923% grain oriented silicon steel or, in some special cases, an amorphous magnetic ribbon) in the form of thin laminations (to minimize eddy current loss) and
(b) Directing magnetic flux flow along the easy magnetization direction throughout most of the core (except the joint sections).
There are two basic techniques to make the low-loss core:
(a) Stacking the laminates, to get a rectangular closed circuit with joints between the core elements, i.e. legs and yokes.
(b) Winding the magnetic strip into a toroidal loop with a specially cut joint, which allows one to open and close the core (after it has been shaped and annealed) to assemble it with the windings. In a stacked core, the magnetic material is not affected by plastic deformation except in a very limited area along the cut edges, so that additional power loss is generated mainly in the joints. In a well stacked single phase core, the effect of joints on core loss increase is  greater than 3%, while in a 3-phase core it is  greater than 10%. In case of a wound core, the entire length of the slit magnetic material is deformed and a stress relief anneal is needed (even in a single phase core), to avoid high power loss in the core ( greater than 15% increase of core loss).
The main drawbacks of a stacked core are the inevitable loss of expensive magnetic material (to make the joints) and complexity of precision stacking, which typically requires manual labor; while the stacked core benefits from lower core loss in the case of a 3-phase transformer and a possibility to fully optimize the core geometry.
In a wound core, the main drawbacks are: stress-relief anneal and the need for a special tooling to keep the core shape during anneal, which limits the optimization of the core dimensions, defined by the tooling dimensions. It should be noted that without stress relief anneal, core loss of the wound core is 15 to 40% higher than the core loss after stress relief. In addition, the magnetic material with highest permeability and lowest loss values (laser scribed domain refined steel) cannot be effectively used in a conventional wound core, because the effect of laser scribing is canceled by the stress relief anneal. The main benefit of a wound core is a much better use of magnetic material since 5 to 15% less material is needed to produce a 3-phase core for small power transformer and there""s no scrap, which in a stacked core is  greater than 5%.
It is an object of the present invention to provide a method of manufacturing a magnetic transformer core which combines the main benefits of both the stacked and wound cores, while it eliminates their main drawbacks. The present invention provides a method of producing a scrapless core which does not require stress-relief anneal and can have filly optimized dimensions, while at the same time provides for minimum core loss, which is equal to or less than for a fully annealed wound core.
In accordance with the present invention there is provided a method of making transformer core laminates with bent corners from magnetic strip material having a predetermined thickness and power loss in the manufacture of a low-stress polyhedral core for a power transformer. The method includes mechanically bending corners in each laminate about predetermined bending lines while limiting the zone in each corner where the laminate is subject to plastic deformation to  less than 5d where d=laminate thickness, so that the specific power loss in the transformer core will equal that of the magnetic strip material except within the zone, where the power loss is higher due to the plastic deformation of the magnetic strip material.
Further in accordance with the present invention there is provided a method of making transformer core laminates with bent corners from magnetic strip material having a predetermined thickness and power loss in the manufacture of a low-stress polyhedral core for a power transformer including the steps of cutting a strip of magnetic material to a predetermined length corresponding to one-half the length of a single turn of the core and reflecting the position of the turn in the core to form a rectangular half-laminate, positioning the half-laminate between a male die and a female die at a bending station, moving the male die toward the female die and against the half-laminate so that a first bend in the first corner is made about a predetermined bending line and at predetermined angle, advancing the half-laminate through the bending station to reach a position for the formation of a second corner in the laminate and moving the male die toward the female die and against the half-laminate so that the first bend in the second corner of the laminate is made about a predetermined bending line and at a predetermined angle, and during the bending of each corner, limiting the zone in each corner where the laminate is subject to plastic deformation to  less than 5d, where d=laminate thickness, so that the specific power loss in the transformer core will equal that of the magnetic strip material except within the zone, where the power loss is higher due to the plastic deformation of the magnetic strip material.
Further in accordance with the present invention there is provided a method of making transformer core laminates with bent corners from magnetic strip material having a predetermined power loss in the manufacture of a low-stress polyhedral core for a power transformer including the steps of mechanically bending corners in each laminate about predetermined bending lines while limiting the plastic deformation to xc2x11.5 mm from each bending line so that the specific power loss in the transformer in the transformer core will equal that of the magnetic strip material except within xc2x11.5 mm from the bending lines, where the power loss is higher due to plastic deformation of the magnetic strip material. In one aspect of the invention each corner of each transformer core laminate is produced by subjecting the laminate to one step of deformation by bending to produce a full 90xc2x0 corner, comprised of one 90xc2x0 bend. In another aspect of the invention each corner of each transformer core laminate is produced by subjecting the laminate to two steps of deformation by bending to produce a full 90xc2x0 corner, comprised of two 45xc2x0 bends. In another aspect of the invention, each corner of each transformer core laminate is produced by subjecting the laminate to three steps of deformation by bending to produce a full 90xc2x0 corner, comprised of three 30xc2x0 bends.
In accordance with another aspect of the invention there is provided a method to produce transformer core laminates consisting of two pieces, half-laminates, each having two right corners consisting of 1xc3x9790xc2x0, 2xc3x9745xc2x0, or 3xc3x9730xc2x0 bends, so that a closed turn is produced with a butt joint between the ends of the two half-laminates.
Further in accordance with the invention, there is provided a method of making transformer core laminates with bent corners from magnetic strip material having a predetermined power loss in the manufacture of a low-stress polyhedral core for a power transformer including the steps of cutting a strip of the magnetic material to a predetermined length corresponding to one-half the length of a single turn of the core and reflecting the position of the turn in the core to form a rectangular half-laminate, positioning the half laminate between a male die and a female die at a bending station, moving the male die toward the female die and against the half-laminate so that a first bend in a first corner is made about a predetermined bending line and at a predetermined angle, advancing the half-laminate through the bending station to reach a position for the formation of a second corner in the laminate and, moving the male die toward the female die and against the half-laminate so that the first bend in the second corner of the laminate is made about a predetermined bending line and at a predetermined angle, and during the bending of each corner limiting the plastic deformation to xc2x11.5 mm from each bending line so that the specific power loss in the transformer core will equal that of the magnetic strip material except within xc2x11.5 mm from the bending lines, where the power loss is higher due to plastic deformation of the magnetic strip material. Further in accordance with the invention the corners are formed so that at no time the convex tip of the bend comes into direct contact with the female part of the die and no part of the laminate is simultaneously in direct contact with the male and female parts of the die.
For a more detailed disclosure of the invention and for further objects and advantages thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings.